The background of the invention will be set forth in two parts.
1. Field of the Invention
This invention relates to surface acoustic wave devices and more particularly to such devices providing fractional efficiency coupling between two tracks of propagating surface acoustic wave energy.
2. Description of the Prior Art
The basic concept of a surface acoustic wave device generally referred to as a multistrip coupler was first introduced by F. G. Marshall, C. O. Newton and E. G. S. Paige while working with the Royal Radar Establishment, Great Malvern, Worcestershire, England. The design and potential usefulness of this type of device is set forth by them in articles in such publications as IEEE Transactions, Microwave Theory & Techniques, MTT-21, 206 (1973) and IEEE Transactions, Microwave Theory & Techniques, MTT-21, 216 (1973).
Most of the multistrip components described in these and similar articles require coupling efficiencies of -3 dB or less, and take advantage of the 90.degree. phase shift between the coupled and uncoupled output ports to provide signal cancellation at specific, selected ports. These components include the reflecting track changer, the echo trap, and others.
The basic problem with these fractional efficiency multistrip couplers is that they are intrinsically narrowband. This results because the maximum in frequency response occurs at the stopband frequency. At practical operating frequencies in the vicinity of one-half the stopband frequency, the coupling efficiency varies rapidly and monotonically with frequency.
As will be subsequently described in greater detail, geometrical parameters for synthesizing fractional efficiency multistrip type surface acoustic wave couplers having broad, symmetrical passbands have now been formulated utilizing a field theoretical model. A description of this model may be found in a doctoral dissertation by Charles Maerfeld entitled "Coupleurs Directifs et Amplificateurs D'ondes Acoustiques de Surface a Bandes Metalliques Multiples," University of Nice, France, June, 1973; and an article entitled "Perturbation Theory for the Surface Wave Multistrp Coupler" by Charles Maerfeld and P. Tournois, in Electronics Letters, Vol. 9, pages 115 and 116, March 1973.
The present invention, utilizing the aforementioned formulation, has been used to determine the significant geometrical alterations of the basic multistrip coupler configuration required to make broadband couplers. The significant change is that the strip period p.sub.1 in the input track differs from the strip period p.sub.2 in the output track. By appropriately adjusting the ratio p.sub.1 /p.sub.2 and properly selecting N, the number of strips, broadband couplers with any desired transfer efficiency less than unity may be designed.
In an article in Electronics Letters, Vol. 9, page 432, (1973), Charles Maerfeld and G. W. Farnell describe an acoustic beam compressor using a multistrip coupler having different strip periods. Although somewhat similar in appearance to the present invention, the device described was for only providing total energy transfer from adjacent tracks of unequal width. Devices with 100% coupling efficiency, as in the acoustic beam compressor, do not possess a frequency response problem.